1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, a method of manufacturing the photoreceptor, and an electrophotographic apparatus and a process cartridge using the electrophotographic photoreceptor. More particularly, the present invention relates to an electrophotographic photoreceptor for use in electrophotographic apparatus such as copiers, facsimiles, laser printers, and direct digital plate making machines, and a method of manufacturing the photoreceptor, and an electrophotographic apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
The photosensitive material used in the photoreceptors for use in an electrophotographic apparatus such as copiers and laser printers has changed from inorganic photosensitive materials such as selenium, zinc oxide and cadmium sulfide to organic photosensitive materials. This is because organic photosensitive materials are friendly to environment, have low manufacturing costs; and good designing flexibility.
Organic photoreceptors are broadly classified into the following three types:    (1) homogeneous single-layered photoreceptors in which, for example, a photoconductive resin such as polyvinyl carbazole (PVK) or a charge transfer complex such as PVK-TNF (2,4,7-trinitrofluorenone) is formed on an electroconductive substrate;    (2) dispersion type single-layered photoreceptors in which a resin layer including a pigment such as phthalocyanine and perylene dispersed in the resin is formed on an electroconductive substrate; and    (3) functionally-separated multi-layered photoreceptors in which a charge generation layer (hereinafter referred to as a CGL) including a charge generation material (hereinafter referred to as a CGM) and a charge transport layer (hereinafter referred to as a CTL) including a charge transport material (hereinafter referred to as a CTM) are overlaid on an electroconductive substrate.
The functionally-separated multi-layered photoreceptors typically have a structure in which a CTL is formed on a CGL. The functionally-separated multi-layered photoreceptors having a reverse structure are sometimes referred to as reverse-layered photoreceptors.
Particularly, the functionally-separated multi-layered photoreceptors have advantages on photosensitivity and good flexibility in designing photoreceptors having high photosensitivity and good durability. Therefore, recently the functionally-separated multi-layered photoreceptors have been widely used for electrophotographic apparatus.
In recent years, a small-sized electrophotographic apparatus which can produce quality images at a high speed has been increasingly demanded. In addition, a polymerized toner having a sphere form and a small diameter (i.e., not greater than 6 μm) now tends to be selected for use in developing images.
To produce high quality images with the demanded high speed rate, the electrophotographic apparatus forms images with high density. This causes an image deterioration problem referred to as “residual image” or “ghost” in many cases. Thus actually there is no perfect electrophotographic apparatus capable of producing high quality images at a high speed as demanded.
The residual image phenomena are now described.
When an image having only distinctive light image portions and dark image portions is formed and then a half-tone image is formed as illustrated in FIG. 9, a residual image (positive or negative image) of the image is observed in the half-tone image in some cases. These images are referred to as “a positive residual image” or “a positive ghost image” (illustrated in FIG. 10) and “a negative residual image” or “a negative ghost image” (illustrated in FIG. 11). It is necessary to prevent formation of such a residual image particularly in a high quality full color electrophotographic apparatus.
The mechanism of formation of a residual image is considered to be caused by fluctuation of the surface potential of the photoreceptor as discussed in published unexamined Japanese Patent Application No. (hereinafter referred to as JOP) 11-133825. The fluctuation of the surface potential of the photoreceptor in each process of latent image formation, development and transfer is explained with reference to FIG. 12.
FIG. 12A illustrates the surface potential of a photoreceptor when the photoreceptor is charged to uniformly have a potential of −700 V and then exposed to imagewise light (i.e., the surface potential of a latent electrostatic image formed on the photoreceptor). In this case, the surface potential of the exposed portion is about 0 V. FIG. 12B illustrates the surface potential of the photoreceptor when the latent image is developed with a toner (i.e., the surface potential of the photoreceptor having a toner image thereon) due to the potential difference between the development potential and the surface of the photoreceptor. FIG. 12C illustrates the surface potential of the photoreceptor after the toner image is transferred onto a receiving paper while a reverse bias is applied to the receiving paper. In this case, the exposed portions have a certain positive potential (for example, +10 V in FIG. 17C).
When the photoreceptor is charged after these image forming processes are repeated, the surface potential (for example, −690 V) of the former image portion is lower than that (i.e., −700 V) of the other portions. If a half-tone image is formed on an area including the former image portion and a former non-image portion, the difference in potential between the former image portion and a non-lighted portion is larger than that between the former non-image portion and the non-lighted portion, and thereby a dense image (a positive image) is formed on the former image portion.
As discussed in JOP 2002-123067, the residual image problem also occurs in a digital image forming method in which half tone images constituted of digital dot images are formed as widely used in the inkjet printing methods.
Specifically, the illuminance in a beam spot formed on a photoreceptor to form a latent dot image thereon is not uniform and has a certain distribution in a direction from the center to the periphery of the beam spot. When a beam spot is formed on the former image portion, the resultant latent dot image portion has a larger area than the other latent dot image portions because the potential of the latent dot image portion is biased by, for example, +10 V. Thus, the resultant dot toner image has a larger diameter than that of the dot image in other portions, and thereby the portions of the widened dot image potions are observed to be dense, resulting in formation of a residual positive image. This phenomenon is more apparent in high definition image formation, for example, in image formation with a resolution of 1200 dpi than with a resolution of 600 dpi.
As discussed in JOP 10-177261, fluctuation of the surface potential is considered to be mainly caused by storage of space charges inside a photoreceptor. In attempting to prevent the storage of space charges, the following methods have been disclosed.
(1) Improvement of Outermost Layer of Photoreceptor
JOP 10-115946 discloses a photoreceptor having an outermost layer which comprises a polyarylate resin and which has a dielectric constant not less than 2.3.
JOP 11-184135 discloses a photoreceptor having a photosensitive layer including an azo pigment and an outermost layer including a polyarylate resin. According to the publication, polyarylate resins have high crystallinity, and therefore can orient the CTM included therein to some extent. It is considered that by orienting the CTM and using the specific azo pigment, the charge injection barrier can be decreased and thereby the photo-memory property of the photoreceptor is diminished.
JOP 10-177263 discloses that a photoreceptor having a CGL including a phthalocyanine compound and an outermost layer including a bisphenol-based polycarbonate is used for an electrophotographic apparatus having an intermediate transfer medium. It is considered that the effect is produced by the combination of the specific compounds.
JOP 10-177264 discloses that a photoreceptor having a CGL including a phthalocyanine compound and an outermost layer including a charge transport polymer is used for an electrophotographic apparatus having an intermediate transfer medium. It is considered that the effect is produced by the combination of the specific compounds.
JOP 10-177269 discloses that a photoreceptor having a CGL including a phthalocyanine compound and either an insulating outermost layer or a semiconductive outermost layer including at least a resistance controlling agent is used for an electrophotographic apparatus having an intermediate transfer medium. It is considered that the effect is produced by the combination of the specific compounds.
JOP 2000-147803 discloses a photoreceptor in which a polycarbonate copolymer obtained from bisphenol A and a monomer having a specific arylene group is used for the outermost layer thereof such as the CTL. It is discussed in the publication that injection of charges having a reverse polarity from the outermost layer side can be prevented.
JOP 2001-235889 discloses a photoreceptor having an outermost layer including a surface-treated particulate metal oxide, an alcohol-soluble resin and an alcohol-soluble CTM. It is described in the publication that thermoplastic resins cannot be used as the binder resin of the outermost layer because the resins have insufficient mechanical strength and solvents used for dissolving the resins also dissolve the photosensitive layer. It is considered that use of an alcohol-soluble CTM prevents formation of residual images.
JOP 2002-6528 discloses a photoreceptor having a photosensitive layer and a protective layer including at least one of an alkali metal element and an alkaline earth metal element. It is described therein that by including such an element in the protective layer, ionic conduction properties can be imparted to the protective layer, and thereby a photoreceptor which has good durability and which does not store residual charges can be provided. It is also described therein that it is possible to reduce the residual charges by including a CTM in the protective layer but the abrasion resistance of the protective layer is weak.
(2) Improvement of Photosensitive Layer
JOP 2000-75521 discloses a photoreceptor including at least one of a chlorogallium phthalocyanine compound and a hydroxygallium phthalocyanine compound as a CGM and a CTM having a hydrazone skeleton. It is described in the publication that the combination of the specific CGM and CTM can diminish the transfer memory property and photo-memory property of the photoreceptor.
JOP 2000-105478 discloses a photoreceptor having a photosensitive layer including an azo pigment which is for use in an electrophotographic apparatus using a laser diode emitting light with relatively short wavelength of from 380 to 500 nm as image writing light. It is considered that the azo pigments used therein have relatively weak photo-memory property compared to α-titanylphthalocyanine.
JOP 2001-305762 discloses a photoreceptor including a CGM and a CTM, wherein the CTM comprises a first compound having a polarizability greater than 70 Å which is calculated by structure optimizing calculation using semiempirical molecular orbital calculation using PM3 parameter and having a dipole moment less than 1.8 D which is calculated by the structure optimizing calculation, and a second compound having 50% transmittance at a longer wavelength than the first compound. It is described therein that the second compound absorbs extra light irradiating the photoreceptor, and thereby the photo-memory property of the photoreceptor can be diminished.
(3) Improvement of CTL
JOP 7-92701 discloses a multi-layered photoreceptor in which an oxytitanium phthalocyanine is included in the CGL and at least two kinds of CTMs are included in the CTL, wherein the difference in oxidation potential between the at least two kinds of CTMs is not greater than 0.04 V. It is considered that by using CTMs having almost the same energy level, hopping of the charge carriers between the CTMs can easily occur and the chance of trapping of charge carriers by the CTMs can be decreased, thereby decreasing the quantity of electrons excited by reverse charging performed by a transfer device, resulting in prevention of occurrence of the residual image problem.
JOP 08-152721 discloses a photoreceptor which is used for a back-lighting type high speed electrophotographic apparatus in which the exposure-development interval is from 10 to 150 msec, wherein the CTL of the photoreceptor has a charge mobility not less than 1×10−6 cm2/V·sec at an electric field of 2×106 V/cm. It is described therein that when a photoreceptor has a low dynamic photosensitivity, the latent image formation cannot be completed before the start of the developing operation and thereby the potential of the former image portions is increased after repeated use; but by using the technique mentioned above, the dynamic photosensitivity can be improved and thereby the residual image problem can be solved.
JOP 10-177262 discloses a photoreceptor which is for use in electrophotographic apparatus having an intermediate transfer medium and which has a CGL including a phthalocyanine compound and a CTL including a compound selected from triphenylamine compounds and N,N,N′,N′-tetraphenylbenzidine compounds. It is considered that the effect is produced by the combination of the specific compounds.
(4) Improvement of CGL
JOP 06-313972 discloses a photoreceptor in which the thickness of the CGL is increased so as to be not less than 25 μm or the content of a CGM in the CGL is increased so as to be not less than 50% by weight so that a number of charge carriers are trapped in the CGL, to make the resultant ghost image inconspicuous.
JOP 10-69104 discloses a multi-layered photoreceptor having a CGL including a triarylamine compound having a xylyl group. It is described in the publication that a barrier to carrier transportation is formed at the interface between the CGL and CTL, and charge carriers are trapped thereby. Since the trapped carriers decrease the space electric field in the CGL, the potential of a half-tone image portion is not decreased, and thereby a residual image is formed at the portion. By including a CTM (i.e., a triarylamine compound having a xylyl group), the generated carriers are rapidly injected into the CTL and transported therethrough, and thereby accumulation of trapped carriers (i.e., occurrence of the residual image problem) can be prevented.
JOP 10-186696 discloses a photoreceptor having an electroconductive substrate and at least a photosensitive layer and a protective layer located overlying the substrate in this order, wherein the photosensitive layer comprises oxytitanium phthalocyanine having an Cu Kα X-ray diffraction spectrum in which strong peaks are observed at Bragg (2θ) angles of 9.5°, 24.1° and 27.3°. It is considered that the effect can be produced by the specific compound.
JOP 2002-107972 discloses a photoreceptor having a CGL including a hydroxygallium phthalocyanine compound and a butyral resin which serves as the binder resin and which has an acetal group, an acetyl group and a hydroxyl group, wherein the butyral resin has a butyralation degree not less than 62% by mole, a weight average molecular weight (Mw) not less than 2.0×105 and a number average molecular weight not less than 5.0×104. It is considered that the number of photo-carriers can be reduced by the specific polyvinyl butyral, and thereby occurrence of the residual image problem can be prevented.
(5) Improvement by Matching CGL with CTL
JOP 07-43920 discloses a multi-layered photoreceptor in which a specific azo pigment is used for the CGL and a CTM having a fluorene skeleton is used for the CTL. It is considered that addition of the specific compounds prevents the photoreceptor from suffering light fatigue. It is considered that the effect can be produced by the specific compound.
JOP 09-211876 discloses a negative polarity-type photoreceptor having a high γ property, in which a CGL including a phthalocyanine compound and a p-type CTL including a material selected from the group consisting of inorganic p-type semiconductors and particulate t-Se and charge transport polymers are used. It is described therein that the p-type CTL is characterized by including no positive hole transport material and thereby diffusion of a positive hole transport material into the CGL can be prevented. Therefore, trapping caused by the phthalocyanine pigment can be prevented and thereby the residual image problem can be prevented.
(6) Improvement of Undercoat Layer
JOP 08-22136 discloses a photoreceptor having an undercoat layer including a silane coupling agent and an inorganic filler. It is described therein that by forming such an undercoat layer, charges to be flown to the substrate are smoothly flown to the substrate, and thereby occurrence of the residual image problem can be prevented.
In addition, JOP 11-184127 discloses a photoreceptor having an undercoat layer including a resin having a specific polyamide acid or polyamide acid ester structure or a polyimide structure, and a resin having a cyanoethyl group. It is considered that by using such resins, the photoreceptor is prevented from suffering light fatigue.
JOP 2000-112162 discloses a photoreceptor having an undercoat layer including a crosslinking resin which hardly changes its resistance even when the environmental humidity changes. It is described therein that JOP 08-146639 discloses an undercoat layer including a polycyclic quinone, perylene, etc.; JOP 10-73942 discloses an undercoat layer including a metallocene compound, an electron accepting compound and a melamine resin; JOP 08-22136 discloses an undercoat layer including a particulate metal oxide and a silane coupling agent; and JOP 09-258469 discloses an undercoat layer including a particulate metal oxide having a surface treated with a silane coupling agent.
It is described therein that in a high sensitive photoreceptor including oxytitanium phthalocyanine in its CGL, a large number of molecules and carriers are excited, and therefore there is a large number of molecules which do not cause charge separation; in addition a large number of electrons and holes tend to remain in the photoreceptor in an electrophotographic process in which charging and light irradiating are repeated.
In attempting to solve the problem, JOP 2000-112162 proposes to use a combination of a polyamide resin and a zirconium compound or a combination of a polyamide resin, a zirconium alkoxide and a diketone compound such as acetyl acetone for the undercoat layer. In addition, JOP 2001-51438 proposes to use a combination of a cellulose resin, a zirconium compound, a zirconium alkoxide, and a diketone compound for the undercoat layer.
JOP 2001-305763 discloses a photoreceptor having an undercoat layer, including a CGM and a CTM, wherein the CTM comprises a material having a polarizability greater than 70 Å which is calculated by structure optimizing calculation using semiempirical molecular orbital calculation using PM3 parameter and having a dipole moment less than 1.8 D which is calculated by the structure optimizing calculation, and a specific arylamine compound, wherein the undercoat layer comprises a particulate titanium oxide treated with an organic silicon compound and a polyamide having a specific diamine component as a constituent. It is considered in the publication that by forming such an undercoat layer, the carriers remaining in the photosensitive layer can be easily transported.
JOP 2002-107983 discloses a system in which the undercoat layer of the photoreceptor has a volume average resistivity of from 1010 to 1012 Ω·cm, the CTL thereof has a thickness not greater than 18 μm and the electrophotographic apparatus does not include a quencher. It is considered that by not using a quencher, the photoreceptor is prevented from suffering light fatigue, and since the undercoat layer has a proper resistance, injection of charges from the substrate to the photosensitive layer can be suppressed, resulting in prevention of accumulation of space charges in the photoreceptor.
(7) Addition of Additives
JOP 10-177261 discloses a photoreceptor for use in an electrophotographic apparatus having an intermediate transfer medium, wherein the photoreceptor has a CGL including a phthalocyanine compound and an outermost layer including a material having a hindered phenol structure. It is considered that the effect is produced by the specific material.
JOP 2000-292946 discloses a photoreceptor having a CGL including a phthalocyanine pigment and a dithiobenzyl compound. It is described therein that by using such materials, the photo-memory property of the photoreceptor can be diminished and thereby occurrence of the positive-ghost problem can be prevented.
(8) Improvement in Electrophotographic Process
JOP 07-13374 proposes a technique in that the photoreceptor used is sometimes charged reversely so as to have a reverse (positive) charge, and then allowed to settle.
In a photoreceptor having a high sensitive CTL, a large number of charge carriers are induced by light irradiation. In this case, electrons whose number is the same as that of the holes injected to the CTL are formed. If the electrons are not discharged to the substrate, the electrons remain in the CGL and thereby the residual image problem tends to occur. When such a photoreceptor is reversely (i.e., positively) charged, electrons are injected from the substrate and electron traps are formed in the CGL. When light irradiation is performed on such a photoreceptor, difference in the number of electron traps between the lighted portions and unexposed portions is little, and thereby the ghost image becomes inconspicuous.
JOP 07-44065 discloses a technique in that a DC voltage overlapped with an AC voltage is applied to the substrate of the photoreceptor. By applying a reverse bias to the substrate, electrons trapped in the CGL can be discharged therefrom. By overlapping an AC voltage, the electric current can be increased and thereby the reverse charge bias effect can be accelerated.
JOP 10-123802 discloses a technique in that charging (not main charging) is performed on a multi-layered photoreceptor having a CGL including a phthalocyanine compound and then light discharging is performed thereon, wherein the main charging is performed thereon if the predetermined portion of the photoreceptor reaches the main charging portion. It is described therein that by performing such an image forming method, the photoreceptor is charged after the space charges in the photoreceptor are released therefrom, and thereby occurrence of the residual image problem can be prevented.
JOP 10-123855 discloses a technique in that a controller is provided in an electrophotographic apparatus, which controls the transfer current flowing from a transfer device to the multi-layered photoreceptor used, wherein the photoreceptor has a CGL including a phthalocyanine compound. It is described therein that the greater the transfer current, the more conspicuously a negative residual image is formed. The reason is considered as follows. When an image is transferred, holes are injected into non-lighted portions of the photoreceptor and the holes are trapped at the interface of the CGL or the CTL on the substrate side. The trapped holes are released in the next charging process, and thereby the dark decay is enhanced (i.e., apparent sensitization), resulting in occurrence of formation of a negative residual image. Therefore, by controlling the transfer current, the number of charge carriers injected into the photoreceptor can also be controlled and occurrence of the residual image problem can be prevented.
JOP 2000-231246 proposes a technique in that the wavelength of the image writing light and the discharging light are determined depending on the ratio of photo-memory property before charging to the photosensitivity of the photoreceptor.
JOP 10-123856 proposes a technique in that light irradiation is performed on a photoreceptor having a CGL including a phthalocyanine compound before the transfer process to decrease the potential of the unexposed portion to ⅓ of the potential, in order to prevent occurrence of the residual image problem. It is considered that by performing such irradiation, the difference in potential between the exposed portion and the unexposed portion is reduced and thereby the residual image becomes inconspicuous.
JOP 10-246997 discloses a technique in that in an electrophotographic apparatus using a photoreceptor having a photosensitive layer and a protective layer including a light-curable acrylic resin, a humidity sensor is provided in the vicinity of the photoreceptor to change the current of the AC component of the voltage applied by the charger depending on the humidity. It is described therein that by using such a technique, chance of formation of blurred images can be decreased. In addition, it is described therein that the photo-memory property of the photoreceptor is weakened using the technique, but the mechanism thereof is not described therein.
JOP 2001-117244 discloses a technique in that in order to prevent formation of ghost images when a S-form photoreceptor is used, the period of half-decay of the potential on the photoreceptor in light irradiation, which period is determined using a Xerographic Time Of Flight (TOF) method, is controlled so as to be not greater than 1/10 of the exposure-development interval between the light irradiation process and the following development process.
As described in numbered paragraph (6) above, JOP 2002-107983 discloses a technique in that by not using a quencher, the photoreceptor is prevented from suffering light fatigue.
JOP 2002-123067 discloses a technique in that the photoreceptor and charging conditions are controlled so as to satisfy the following relationship: |(V1−V2)/VH|<0.020, wherein VH represents the potential of the charged photoreceptor; V1 represents the potential of the photoreceptor after a dark decay for a time of 10T, wherein T represents the charge-exposure interval; and V2 represents the potential of the photoreceptor after a dark decay for a time of 10 T, which photoreceptor is charged again after one round of charging followed by light irradiation has been completed. Specifically, a technique is described therein that the process speed is increased to have a short dark decay time or the photoreceptor is charged so as to have a relatively low potential.
Although the techniques described above are applied to prevent occurrence of residual images, it is found that these are not good enough to obtain an electrophotographic device having good durability and producing high quality images at a high speed. Thus, the residual image problem has not been fully solved.
Because of these reasons, a need exists for an electrophotographic apparatus which can produce high quality images at a high speed with good durability, good cleanability and good transferability while obviating the residual image problem.